1. Field of the Invention
The present invention relates to a process and apparatus for separation of hydrocarbons, used for separating and recovering hydrocarbons such as ethane, propane, butane and the like from a liquefied natural gas.
2. Background of the Invention
It is being conducted generally to liquefy a natural gas at about −162° C. at around atmospheric pressure, send the liquefied natural gas to a marketplace by marine transportation, vaporize the liquefied natural gas, and then feed it into a natural gas pipeline as a town gas or a fuel for thermal power generation. Incidentally, a natural gas liquefied at around atmospheric pressure is called liquefied natural gas (LNG). The liquefied natural gas received at the marketplace contains, in some cases, a large amount of hydrocarbons composed of ethane, propane, butane and a small amount of heavier components. Such a liquefied natural gas has a high calorific value and therefore may not meet the natural gas specification required by the marketplace. Or, the hydrocarbons such as ethane, propane, butane and the like, contained in liquefied natural gas can be used as a raw material for petrochemical plants and therefore have, in some cases, a higher commercial value than when used as a town gas or as a fuel for thermal power generation. Hence, it has been desired to separate and recover hydrocarbons such as ethane, propane, butane and the like from a liquefied natural gas received by the marketplace before the natural gas is fed into a natural gas pipeline.
In order to separate and recover, from a liquefied natural gas, a fraction containing components heavier than methane, it is necessary to vaporize a major portion of the liquefied natural gas. Therefore, the reboiler of a demethanizer needs a large amount of heat. The feeding of heat to the reboiler of the distillation column is generally conducted by a method of direct heating using a heat medium such as steam or by a method wherein a thermal medium such as hot oil is circulated. In any method, however, generation of steam or heating of oil is necessary, consuming a large amount of a fuel.
In order to reduce the heat amount required by the reboiler of the demethanizer, it has been known to heat the liquefied natural gas fed to the demethanizer, by utilizing the sensible heat of air. By replacing part of the heating by the reboiler with heating by air, it is possible to reduce the amount of the fuel consumed for the reboiler of the demethanizer. However, the air-heated heater requires a very large heat transfer area for heat exchange; further, a structure for supporting the heater and a sufficient space for the structure are required; therefore, an improvement in investment cost is desired.
Meanwhile, in a process for separating and recovering, from a liquefied natural gas, a fraction containing components heavier than methane, the heavy fraction obtained from the bottom of the demethanizer need be separated into products such as ethane, propane and butane, for utilization of these individual products. Accordingly, there are installed, in addition to the demethanizer, a deethanizer, a depropanizer, etc.
In Non-patent Literature 1 or Non-patent Literature 2, it is described that, in such a process, a liquefied natural gas is introduced into the condenser (overhead condenser) of a deethanizer to conduct heat exchange between the liquefied natural gas and the overhead gas of the deethanizer, whereby the condensation of the overhead gas and the heating of the liquefied natural gas are simultaneously conducted.    [Non-patent Literature 1] J. Mark et al., “LNG Flexibility”, Hydrocarbon Engineering, October 2003    [Non-patent Literature 2] C. C. Yang et al., “Cost-effective design reduces C2 and C3 at LNG receiving terminals”, Oil & Gas Journal, May 26, 2003